The present invention generally relates to second-harmonic generation (SHG) elements and methods of producing the same, and more particularly to a waveguide type SHG element which has a high conversion efficiency, a small deviation in the refractive index and a large optical damage threshold.
Recently, lasers and semiconductor lasers in particular are popularly used in apparatuses such as laser printers and laser scanners, and are also used as as a light source for emitting a laser beam which writes and/or reads information on and/or from an optical disk. On the other hand, there are demands to reduce the wavelength of the laser beam, from infrared light to visible light, for example, so as to increase the memory capacity of the optical disk, facilitate the handling of the laser beam and the like. However, although there are developments to reduce the wavelength of the semiconductor laser, it is extremely difficult to reduce the oscillation wavelength to 600 nm or less using the present technology. For this reason, there are demands to realize devices which can obtain coherent light having a short wavelength using the SHG, and particularly to realize a method of producing an periodically domain-inverted optical waveguide for use in such devices.
Conventionally, there is a well known SHG element which passes a laser beam through a nonlinear optical crystal bulk. For example, LiNbO.sub.3 which is a ferroelectric crystal is cut into blocks, and both side surfaces of the block are subjected to an optical polishing to form laser beam input and output surfaces. When a laser beam having an angular frequency .omega. is input from one side surface of the block, a second harmonic having an angular frequency 2.omega. which is twice the angular frequency .omega. is generated from the other side surface of the block. A conversion efficiency .eta..sub.SHG in this case can be described by P.sub.2.omega. /P.sub..omega., where P.sub.2.omega. denotes the power of the second harmonic and P.sub..omega. denotes the power of the fundamental wave. As is well known, the second-harmonic power P.sub.2.omega. has a sin.sup.2 characteristic. The second-harmonic power P.sub.2.omega. deviates periodically with the sin.sup.2 curve because refractive indexes n.sub..omega. and n.sub.2.omega. with respect to the respective frequencies differ, where n.sub..omega. denotes the refractive index of the fundamental wave and n.sub.2.omega. denotes the refractive index of the second harmonic. In other words, the phase of the second harmonic generated at each point do not match due to the difference in the refractive indexes, and the second harmonic deviates due to the phase error with a period corresponding to a phase difference of 2.pi.. In normal crystals, the difference between n.sub..omega. and n.sub.2.omega. is large due to the wavelength dispersion of the refractive index. Accordingly, a coherent length l.sub.c which corresponds to half the deviation period of the second harmonic is extremely small, and the second-harmonic power P.sub.2.omega. also becomes extremely small. The quasi phase matching (or index matching) has been proposed to eliminate this problem.
For example, Tada et al., "Introduction to Optical Electronics" 3rd Edition, Maruzen Co., Ltd., 1988, pp. 262-265 proposes a method of obtaining a large second-harmonic power by satisfying the so-called phase matching conditions in which a laser beam is input so that a refractive index n.sub.e of extraordinary light of a second-harmonic light .lambda..sub.2 matches a refractive index n.sub.0 of ordinary light of a fundamental wave light .lambda..sub.1.
FIG. 1 shows a second-harmonic output characteristic for a case where the phases are matched. In FIG. 1, the ordinate indicates the second-harmonic power P.sub.2.omega. and the abscissa indicates the crystal length 1. A characteristic 3 indicated by broken lines show a case where the refractive indexes of the second-harmonic light .lambda..sub.2 and the fundamental wave light .lambda..sub.1 are matched and the second-harmonic power P.sub.2.omega. increases with increasing crystal length 1. However, in the case of the bulk crystal type described above, the phase matching conditions must be satisfied, and furthermore, the light intensity of the fundamental wave must be made large since the obtained crystal does not have a large nonlinear optical coefficient. For these reasons, the method of obtaining the large second-harmonic power has not been reduced to practice for a low-power light source such as the semiconductor laser.
On the other hand, methods of obtaining a large conversion efficiency .eta..sub.SHG using an optical waveguide type element with quasi phase-matching have recently been proposed. For example, it is possible to utilize the large nonlinear optical coefficient by matching the phases on the curve of the refractive index n.sub.e of the extraordinary light, so that the large conversion efficiency .eta..sub.SHG is obtained. In this case, the refractive indexes of the fundamental wave light and the second-harmonic light differ.
FIG. 2A shows an example of an optical waveguide type SHG element in a perspective view, and FIG. 2B shows a cross section of this SHG element taken along a line X--X' in FIG. 2A.
In FIGS. 2A and 2B, a +Z face of a LiNbO.sub.3 substrate 1' is optically polished so that it is possible to obtain a polarization direction and an incident direction such that the largest nonlinear optical coefficient can be obtained. An periodically domain-inverted optical waveguide 4' is formed on top of the substrate 1'. As shown in FIG. 2B, when the ferroelectric upward domains are arranged on the substrate 1' and regions 30' having a predetermined depth with the downward domain inversions are formed at a constant pitch at the part of the optical waveguide 4' with a period .LAMBDA., for example, it is possible to obtain the periodically domain-inverted optical waveguide 4' in which the downward domain inversion regions 30' are arranged at the constant pitch.
It is known from Armstrong et al., "Interactions between Light Waves in a Nonlinear Dielectric", Physical Review, Vol. 127, 1962, pp. 1918-1939 that a large second-harmonic power can be obtained when a laser beam having an angular frequency .omega. is input to the periodically domain-inverted optical waveguide 4' from the left in FIG. 2B so as to satisfy the quasi phase matching conditions and output to the right.
In FIG. 1, a characteristic 1 shows an ideal second-harmonic output characteristic for the case where the phases are matched. In this case, the second-harmonic power P.sub.2.omega. increases with increasing crystal length 1, and in addition, the conversion efficiency is greatly improved compared to the bulk crystal type because a large nonlinear optical coefficient is used.
In order to make the periodically domain-inverted optical waveguide 4' described above, it is necessary to partially form the domain inversion regions, such as the downward domain inversion regions 30' shown in FIG. 2B. There basically are two known methods of forming such domain inversion regions.
According to a first method, a suitable mask is provided on the substrate 1' and the substrate 1' is then heated to a high temperature. In this case, the Li at the exposed surface parts of the substrate 1' diffuses to the outside and the domain at these parts becomes inverted. However, the depth of the domain inversion region which is formed is only on the order of 1 .mu.m. In addition, there is a problem in that the refractive index varies at these parts, thereby making the optical waveguide unsuited for practical use.
On the other hand, according to a second method, Ti is deposited on parts of the substrate 1' where the domain inversion regions are to be formed, and the substrate 1' is then subjected to a thermal process. In this case, the domain at these parts becomes inverted, and the depth of the domain inversion region which is formed can be made to several .mu.m.
Next, a description will be given of an example of a conventional method of forming the partial domain inversion regions, by referring to FIGS. 3A through 3E.
In FIG. 3A, the +Z face of a single domain ferroelectric crystal such as LiNbO.sub.3 is optically polished to form a substrate 1'.
In FIG. 3B, a Ti layer 20 is formed to a thickness of 300 nm on the substrate 1, by a vacuum evaporation.
In FIG. 3C, a photoetching process is carried out so that the Ti layer part where the domain inversion regions 30' are to be formed remains, thereby forming a Ti layer pattern 20'.
In FIG. 3D, the substrate 1' is subjected to a thermal process at 1000.degree. C. to diffuse the Ti. As a result, Ti diffusion regions 20" are formed, and the domain inversion occurs at the surface portion of the substrate 1' at the Ti diffusion regions 20".
In FIG. 3E, an appropriate fluid is used to clean the surface of the substrate 1' so as to remove any residual Ti which may still remain at the surface part of the substrate 1'. As a result, the partial domain inversion regions 30' which form the domain inversion regions of the periodically domain-inverted optical waveguide are formed, and the waveguide type SHG element can be produced by forming an optical waveguide on partial domain inversion regions 30' of the substrate 1'.
However, the partial domain inversion regions 30' which are formed by the above described method includes Ti. For this reason, as is well known in the art, the so-called optical damage threshold decreases when a laser beam is passed through the partial domain inversion regions 30', and furthermore, the refractive index changes. As a result, the second-harmonic power of the periodically domain-inverted optical waveguide does not increase beyond a certain point as indicated by a dotted line 2 in FIG. 1, and the loss caused by scattering light due to the change in the refractive index increases. In addition, there are problems in that the method described above cannot be applied to crystals having a relatively low Curie point, such as LiTaO.sub.3 and potassium titanium phosphate (KTP).